See logging options in Admin / System settings, bottom of screen.
About SMTP logging, if you really want more, you can use the commented-out lines in this function
https://github.com/salesagility/SuiteCRM/blob/master/include/SugarPHPMailer.php#L438
The lines on top of the function are good to check parameters being sent. The ones at the bottom throw full SMTP logs into the log, these are good for troubleshooting server connections, authentication, etc.
Note that the app already includes ways to track email sent in Campaigns, without any need to edit code. So I am assuming you know this but you have some extra requirement.